1. Field of the Invention
This invention relates to a method of curing printing blankets and printing blankets produced thereby, and in particular relates to a compressible printing blanket of the type used in offset lithographic printing.
2. Prior Art Statement
The use of blankets in offset lithography is well known and has a primary function of transferring ink from a printing plate to paper. Printing blankets are very carefully designed so that the surface of the blanket is not damaged either by the mechanical contact of the blanket with the parts of the press or by chemical reaction with the ink ingredients. Repeated mechanical contacts cause a certain amount of compression of the blanket which must be within acceptable limits so that the image is properly reproduced. It is also important that the blanket have resiliency, i.e. be capable of eventually returning to its original thickness, and that it provide constant image transfer regardless of the amount of use to which it is put.
Printing blankets are normally composed of a substrate base material which will give the blanket integrity. Woven fabrics are preferred for this base. The base may consist of one, two, three, or more layers of fabric. The working surface, by which is meant the surface that actually contacts the ink, is usually a layer of elastomeric material such as rubber. The blanket is conventionally made by calendering or spreading rubber in layers until a desired thickness of rubber has been deposited, after which the assembly is cured or vulcanized to provide the finished blanket. Such a blanket is acceptable for many applications, but often lacks the necessary compressibility and resiliency needed for other applications. It is desirable, therefore, to produce more highly compressible blankets with improved resiliency.
It is difficult to obtain an improved compressibility factor by the standard construction described above because the rubber material, while it is highly elastomeric, is not compressible and cannot be compressed in a direction at right angles to its surface without causing a distortion or stretch of the blanket in areas adjacent to the point of compression. If irregularities exist in the printing plate, the presses, or the paper, the compression to which the blanket is exposed will vary during the operation and the irregularities in the plates, presses or paper will be magnified by the lack of compression in the printing blanket.
The key to providing a printing blanket having the desired compressibility and resiliency is in providing a compressible layer therein.
In particular, it has been found that by including at least one layer of material comprising a compressible layer of resiliant polymer in a printing blanket that printing problems such as those described above as well as "blurring" (lack of definition), caused by a small standing wave in the blanket printing surface adjacent to the nip of the printing press, can be avoided. Also, a compressible layer can serve to absorb a "smash", that is a substantial deformation in the blanket caused by a temporary increase in the thickness in the material to be printed, for example, the accidental introduction of more than one sheet of paper during the printing operation. By incorporating a compressible layer in the blanket, a "smash" can be absorbed without permanent damage to the blanket or impairment of the printing quality of the blanket. In addition, a resilient, compressible layer helps to maintain the evenness of the printing surface and the thickness of the blanket during the printing operation by restoring the normal thickness of the blanket after compression at the nip of the press.
Many different means of producing a compressible layer within a printing blanket are known in the art. For example, compressible layers have been formed by mixing granular salt particles with the polymer used to produce the layer, and thereafter leaching the salt from the polymer to create voids. The voids in the layer make possible positive displacement of the surface layer without distortion of the surface layer since volume compression occurs and displacement takes place substantially prerpendicular to the impact of the press. Such a method is disclosed in Haren et al U.S. Pat. No. 4,025,685. Other methods, such as using compressible fiber structures have been used heretofore to produce compressible layers. Examples are found in Duckett et al U.S. Pat. Nos. 3,887,750 and 4,093,764. Rodriguez, U.S. Pat. No. 4,303,721 teaches a compressible blanket made using blowing agents to create voids in the compressible layer. The use of rubber particles to create voids is disclosed in Rhodarmer U.S. Pat. No. 3,795,568.
The forming of voids using blowing agents has the disadvantages that the size of the voids formed, and the interconnecting of the voids is not easily controlled. Oversized voids and interconnected voids cause some areas of the printing blanket to be more compressible and less resilient than adjacent areas of the printing blanket, which results in deformations during printing. The leaching of salts from a polymer matrix has the disadvantages that the particle sizes used are limited, and that the leaching step is difficult, time consuming and expensive.
More recently, it has been found preferable to produce printing blankets having a compressible layer comprising a cellular resilient polymer having cells or voids in the compressible layer in the form of discrete microcells. It has been found particularly advantageous to produce a compressible layer by incorporating hollow microcapsules in the polymer, as illustrated by Shrimpton et al in U.S. Pat. No. 3,700,541 and corresponding British Pat. No. 1,327,758; and by Larson in U.S. Pat. No. 4,042,743.
In prior art methods of producing a compressible layer for a printing blanket employing microcapsules, it has been found that the thickness of the compressible layer formed is not easily controlled since microcapsules most suitable for this use will melt at a temperature lower than the vulcanizing temperature used for vulcanizing the printing blankets. Since the microcapsules melt before the vulcanization is complete, and before the compressible layer achieves a set structure, agglomeration of the voids created by the microcapsules occurs, and size variations in the voids also occur. This can affect the overall performance properties of the blanket. Also, the variations in the sizes of the voids can weaken the printing blanket and cause the printing blanket to wear out prematurely.